Compression ratio question

[Blaze]

Bare with me because this is making sense in my head but I feel like I'm going to struggle to explain what I'm thinking.

The compression ratio of an engine is a measurement of how much force is applied to the A/F mixture before the spark plug fires and ignites the fuel, yeah? Would a higher compression ratio applied to the same A/F mixture (I.E. everything else on the engine is stock except for the pistons) result in an increased amount of force pushing down on the piston after the spark plug fires? Assuming the rods would be able to handle the increased force that should result in an increase in torque, right?

For example;

B16a2 compression ratio is 10.2:1
If new pistons were put into the engine to increase the compression ratio to, say, 11.2:1 would that mean that the same engine would end up producing more torque?

[bennjamin]

No - the compression ratio tells you what is its largest capacity to its smallest capacity (=1) in the compression of a cylinder.

Essentially increasing displacement encourages more torque.

Try this for more info https://www.google.com.au/#q=compression+ratio+

[Blaze]

From the wiki article:

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature."

https://en.wikipedia.org/wiki/Compression_ratio

So does the above mean that you can increase a car's fuel efficiency by increasing the compression ratio? That's what it means when it says that you can extract more mechanical energy (Power?) from a given mass of A/F mixture. So in the B16A if you increase the compression ratio you'll be able to make the same level of mechanical energy as before but you'd only need to use less fuel?

[rc_]

Compression ratio is the ratio of the swept volume to volume of the cylinder/combustion chamber at TDC.

increasing the compression ratio increases the efficiency of the combustion, meaning for the same intake charge more energy is available compared to when a lower compression ratio is used.

So, you are partly right. In general if you increase your compression ratio you can gain some torque assuming other conditions are optimal (ie ignition/cam timing, fuel type etc).

There is a limit you can increase to and still expect to get net gains without causing damage though, b series has been very widely covered so you will easily find information on what numbers are achievable on the web if you look in the right places.

[rc_]

From the wiki article:

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature."

https://en.wikipedia.org/wiki/Compression_ratio

So does the above mean that you can increase a car's fuel efficiency by increasing the compression ratio? That's what it means when it says that you can extract more mechanical energy (Power?) from a given mass of A/F mixture. So in the B16A if you increase the compression ratio you'll be able to make the same level of mechanical energy as before but you'd only need to use less fuel?

Looks like you just found a decent answer before I posted.

In simple terms yes what you said is possible. Just keep in mind this process is already optimised when the engine was designed. There are always compromises to engine design and tuning (think - emissions regulations, fuel type in use, drivability)

If you're thinking that by increasing the compression ratio on a otherwise stock engine/tune that you will see a significant reduction in fuel consumption I would say no. It would also be a very costly exercise, a cheaper way of getting better fuel economy would be adjusting driving style.

If you're talking about increasing compression ratio coupled with some aftermarket cams and a tune then yeah you might see some torque gains, not necessarily better fuel economy though.

[newpaddy3]

From the wiki article:

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature."

https://en.wikipedia.org/wiki/Compression_ratio

So does the above mean that you can increase a car's fuel efficiency by increasing the compression ratio? That's what it means when it says that you can extract more mechanical energy (Power?) from a given mass of A/F mixture. So in the B16A if you increase the compression ratio you'll be able to make the same level of mechanical energy as before but you'd only need to use less fuel?

basically yeah

but you'd possibly need to run a higher octane fuel

there are also different energy densities in fuels too (not limited to petrol)

so if you're trying to make it more efficient to save money on fuel you won't really achieve that goal

[Blaze]

basically yeah

but you'd possibly need to run a higher octane fuel

there are also different energy densities in fuels too (not limited to petrol)

so if you're trying to make it more efficient to save money on fuel you won't really achieve that goal

At the moment there's no goal regarding compression. I'm trying to get my head around what each change to a car does and I thought I was on to something but no. hahaha.